Convert extrude plane to solid
You may have already solved this, but do this :

1/ Select plane and go into edit mode (tab)

2/ Select All (a)

3/ Activate Extrude mode (e)

4/ Depending on the orientation of your plane, limit extrusion to the X, Y or Z axis by selecting "X", "Y" or "Z".

You plane should now be a solid. Just note that you need to be aware of the normals for your faces you have created with this method. Normals explained here